This application claims the priority of Korean Patent Application No. 2003-7117, filed on Feb. 5, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
Methods and apparatuses consistent with the present invention relate to a handover method in a wireless local area network (WLAN) and a mobile node device that carries out a handover in a WLAN.
2. Description of the Related Art
The Internet has become one of the most important information infrastructures. While the number of Internet users and the amount of Internet equipment has rapidly increased, several problems, such as IP address shortages, low routing efficiency, low mobility, and low quality-of-service (QoS), have appeared. To address these problems, Internet Protocol version 6 (IPv6) has been developed and is now expected to be commercialized.
In addition, mobile IPv6, which can more reliably and more efficiently guarantee mobility while providing typical IPv6 functions, has been developed. However, in order to enable seamless mobile communications, a handover process on Layer 3 (IP layer) is essential.
Recently, an Internet draft (IETF:draft-ietf-mobileip-fast-mipv6-0.5.txt) entitled “Fast Handovers for Mobile IPv6” has been suggested. The Internet draft presents an algorithm capable of carrying out a quick handover with the help of Layer 2 (link layer).
In the meantime, handovers in Layer 3, which is an IP layer, are essential for seamless mobile communications. So far, various algorithms for carrying out fast handover by taking advantage of Layer 2 (i.e., link layer) have been suggested.
FIG. 1 illustrates a network system on a 802.11 WLAN having relay access points (AP). Referring to FIG. 1, the network system includes subnet 1 (110) connecting access routers (AR) 111 through 113 and APs 114 through 118 and subnet 2 (120) connecting ARs 121 through 123 and APs 124 through 128.
A subnet is a layer 3 network with a single shared prefix, and a network adopting a plurality of relay APs based on a WLAN is a layer 2 network.
A mobile node 130 carries out a handover with a plurality of APs while moving from place to place. In a case where the mobile node 130 moves between APs in the same subnet, an L3 (i.e., layer 3) handover is not carried out. On the other hand, in a case where the mobile node 130 moves from an AP in one subnet to an AP in another subnet, an L2 (i.e., layer 2) trigger occurs. Accordingly, the L3 handover is carried out together with an L2 handover.
FIG. 2 illustrates a network system on a 802.11 WLAN including integrated APs/ARs.
Referring to FIG. 2, the network system is based on a single subnet, i.e., a network with a single shared prefix, in which a single AP using a WLAN is used. Accordingly, one AR exists for an AP. Therefore, if a mobile node 240 meets a new AP while moving from place to place, an L2 trigger occurs, and thus an L3 handover is carried out together with an L2 handover.
FIG. 3 illustrates an example of a 802.11 WLAN system. Referring to FIG. 3, a basic service set (BSS) 310 constituted by an AP 313 and clients 311 and 312, and a BSS 330 constituted by an AP 331 and clients 332, 333, 334 and 335, are connected to a distribution system (DS) 320.
A conventional handover method is illustrated in FIG. 4.
When an L2 trigger occurs, a mobile node 400 recognizes that it has just entered a new network. In addition, the mobile node 400 can identify BSS identification (ID) of a new access router (NAR) 420.
Then, the mobile node 400 issues a request for information on the NAR 420 corresponding to the identified BSSID to a previous access router (PAR) 410 by sending an RtSolPr message 430 to the PAR 410.
The PAR 410 searches its distributed service table (DST) for the requested information, for example, prefix information, a link layer address, and an IP address of the NAR 420, and returns the information to the mobile node 400 by sending a PrRtAdv message 440 to the mobile node 400.
The mobile node 400 creates a care-of-address (CoA) using its own media access control (MAC) address and the prefix information of the NAR 420, received from the PAR 410, and transmits a fast binding update (FBU) message 450 to the PAR 410.
The PAR 410 responds to the reception of the FBU message 450 by transmitting an acknowledgement (ACK) message 450 to the mobile node 400.
Thereafter, the PAR 410 transmits a handover initiation (HI) message 460 to the NAR 420, and the NAR 420 transmits an ACK message 160 to the PAR 410 in response to the reception of the HI message 460, thus completing an entire handover process.
If there are packets to be transmitted from the PAR 410 to the mobile node 420 during the handover process, the packets are transmitted to the mobile node 420 via the NAR 420. After the handover process, the mobile node 400 uses the NAR 420.
FIG. 4 illustrates a fast handover process carried out in the network structure shown in FIG. 2. However, an IPv6 network is generally constituted by subnets in each of which a plurality of APs and a plurality of ARs, rather than a single AP, are provided, like the subnets shown in FIG. 1.
In the network of FIG. 1, if the mobile node 130 changes access routers through L2 handover, a process of checking whether or not a NAR belongs to the same subnet that a PAR belongs to is necessary. In the prior art, there are two different types of methods of handing over a mobile node between APs, which will be described in the following paragraphs.
First, in the case of handing over a mobile node between APs in the same subnet, the mobile node is required to communicate with an AR, i.e., an L3 layer, in order to check which subnet a PAR and a NAR belong to. In the case of handing over a mobile node between APs in different subnets, the mobile node is also required to communicate with the AR in order to check whether or not the NAR belongs to the same subnet that the PAR belongs to, and then a fast handover process is carried out, as shown in FIG. 4.
Therefore, when a mobile node changes access routers, there is a need to check whether or not a NAR belongs to the same subnet that the PAR belongs to, which may cause handover latency. Here, handover latency indicates a period of time during which the mobile node is unable to receive or transmit packets over an IP layer due to a handover process.
In the meantime, Korean Patent Laid-open Publication No. 10-2002-0023918 entitled “Fast Hand-Off Method Using Beacon Message Based On Wireless LAN” discloses a handover technique in which an access router, to which a mobile node is to be handed over, periodically sends out a beacon message containing power intensity measurement information and prefix information and a user terminal that receives the beacon message creates an address using the prefix information and carries out a handover operation using the created address. In this technique, an NAR's prefix information is contained in a beacon message, and then the beacon message is transmitted to a user terminal. Therefore, the beacon message needs to be large enough to contain the NAR's prefix information. However, a beacon message, which is large enough to contain prefix information, is hardly compatible with a conventional network system.